Means and method for collecting secondary electrons



March 30, 1937. R. H. VARIAN 2,075,378

MEANS AND METHOD FOR COLLECTING SECONDARY ELECTRONS Filed March 13, 1935 g WWW www alww wwww v INVENTOR, RUSSEL L H. VAR/AN.

kM -WM ATTORNEYS.

Patented Mar. 30, 1937 UNITED STATES PATENT OFFICE Russell H. Varian, San

Francisco, Calif., assignor to Farnsworth Television Incorporated, a corporation of California Application March 13, 1935, Serial No. 10,889

3 Claims.

My invention relates to the collection of secondary electronic emission in electron discharge tubes, as for example, in an image dissector tube adapted to be used in a television transmitter.

5 Among the objects of my invention are: to provide a simple and efiicient means and method for collecting secondary electronic emission in cathode ray tubes; to provide a means and method, in an image dissector tube for television use, for preventing the adulteration, by secondarily emitted electrons, of component parts of a train of electrical impulses; to provide a compound anode for image dissector tubes; to provide a means and method of selecting successive portions, of primary electrons, from a mixed beam of primary and secondary electrons; to provide an image dissector tube, in a television system, capable of producing an undistorted television signal; and to provide a simple and efficient image dissector tube.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a longitudinal sectional view of an image dissector tube embodying my invention.

The various circuits, and component parts thereof,

are shown diagrammatically.

Figures 2 and 3 are diagrammatic views showing modified arrangements of the electrodes in the tube.

Broadly, my invention comprises the combination, in a cathode ray tube having a cathode adapted to discharge a beam of electrons, an anode for collecting selected portions of the beam, and a second anode for collecting other portions 0 of the beam, of means for preventing secondary electronic emission from one of the anodes from reaching the other of the anodes.

The image dissector tube of my invention is an improvement on the image dissector tubes shown s; and described in United States Patent No. 1,773,980, entitled Television system, and issued to Philo T. Farnsworth on August 26, 1930, and in the Farnsworth application, Serial No. 245,334, filed January 9, 1928.

m While such dissector tubes are capable of efficient operation in television systems, it has been found that the output current therefrom may be adulterated, that is, it is not entirely a true representation, in terms of electron density, of the 5 image. This is due in part to the admittance, to

(Cl. 250-2'l.5)

the remainder of the beam being received by the screen. When these latter electrons strike the screen surfaces they liberate a larger quantity of relatively slow moving secondary electrons therefrom. The ratio of secondary to primary emission depends on the material used in the construction of the anode; nickel, a material extensively used for such purposes having a secondary emission ratio of l to 1.4. Another factor which might increase the secondary emission of y the anode is accidental deposition of some of the photosensitive caesium or potassium on the anode during the operation of distilling these materials on the surface of the cathode, as these materials are also good secondary emitters.

Most of those secondary electrons emanating from portions of the screen are attracted back to the positively charged screen again causing no appreciable distortion of the picture current. However, since the electrons follow trajectories, which cause their return to the screen at points remote from their points of origin, it is obvious that a portion of the electrons will fall through the aperture and be collected by the target anode and will flow in the output circuit as distortion currents of such strengh as to seriously impair the operation of the tube.

. I have provided means whereby this objectionable secondary emission may be prevented from entering the picture current circuit, thereby permitting only those electrons originating at the cathode to be collected by the target anode. This allows the transmission of television signals from which images may be reproduced, possessing increased sharpness and brilliancy.

While the device of my invention may be used with the Farnsworth tubes mentioned above, and while I may choose to utilize some or all of the features of these tubes, I prefer to illustrate my invention as being incorporated in a dissector tube having an asymmetrical accelerating field. Such a tube is shown and described by Robert E. Rutherford in his application entitled Image dissector and method of electron beam analysis,

Serial No. 696,999, filed November 7, 1933.

Describing my invention in greater detail, a substantially tubular envelope 2 is provided at one end thereof with a reentrant stem 3 having a pinch I, and at the other end with a planar window 6. A band I is mounted onthe stem and material which is a good emitter of electrons when excited by light, rays, may be distilled in vacuo onto the plate to provide a photosensitive layer l2 on the face of the cathode.

An optical system diagrammatically represented by a lens l8, may be placed adjacent 'the win-' dow 8 so that an optical image is projected on the cathode for the p rp se of instituting an electron flow therefrom. This flow is directed 1 toward an electrical shutter which consists of a sleeve anode l4, sealed into and projecting from the side wall of the envelope, and provided with an aperture l8 facing the cathode. An electrostatic field is created between the cathode and the anode by providing an external source of potential, such as the battery l1, having its positive terminal connected, through the lead l8, with the anode l4 and the negative terminal, through the lead i8,-with the cathode. By means 2 of this field the electrons are accelerated from the cathode to the anode.

A focusing field, similar to the one described above, is provided and comprises a solenoid 2|, disposed about the enevelope, having a battery 22 and a variable resistor 28 in circuit therewith,

by means of which the current through thesolenoid may be varied, for focusing the electron beam in the plane of the shutter.

f Means are provided for deflecting the beam cyclically to cause successive portions thereof to traverse the anode aperture. Coils 24 and 26 are disposed with their respective axes at right angles to each other and to the longitudinal axis of the beam, and each coil is connected to the 40 outputof oscillators 21 and 28 respectively. The current output of these I oscillators preferably has a sawtooth wave form: one of the circuits being capable of operation at from 16 to 60 cycles and the other at from 1 to 5000 cycles. The magnetic fields of the coils, directed at right angles to the beam, causes it to be deflected cyclically in two directions so that each elemental section thereof passes before the anode aperture.

If the electrostatic field were uniform the beam would have elemental cross sectional densities corresponding to elemental light intensities of the image projected on the cathode. In the present tube this condition obtains only in the plane of the cathode; most of the electrons in the beam tending to flow in curvilinear paths to the anode due to the asymmetrical positioning of this elec trode in the tube.

However, under the influence of the various fields, that portion of the beam aligned with the anode aperture, at a given instant, follows a substantially rectilinear path from the cathode and has an electron density corresponding to the light intensity of the spot on the cathode from which it originated. Thus it will be seen that regardless of the path of the electrons in the beam, successive portions thereof traveling in a rectilinear path will be collected, and the resultant signal train will be a true electrical dissection of the image.

Disposed within the anode I 4 is a target anode 28 which is positioned directly behind the aperture It. A lead 8|, connected to the target anode, is brought out through a seal in the envelope wall and is connected to the positive terminal of a battery 82 connected in series with the battery II. This creates a greater positive charge on the target anode 28 than on the anode l4.

As the deflected beam passes before the aperture i8 successive elemental portions thereof pass through and are collected by the positively charged target anode to produce a current in the output circuit 88. All portions of the beam other than the selected'portion, at a given instant. are collected by the anode I4 and passed through the lead l8 andthe battery 82 to the output circuit. A resistor 84 is provided in series with the-lead 8| and the current flowing in this resistor is a function of the difference in the number of electrons collected by the target anode and the anode ll. This current may be taken from the circuit 88, across which the resistor 84 is shunted, and amplified for transmission by wire or radio.

As the highly accelerated electrons in the beam strike the sleeve anode and the target anode, they liberate multiplied slow moving secondary electrons. Some of these electrons will normally return to the positively charged sleeve anode, and, as stated above, some of the electrons, due to their trajectory, will fall through the aperture and be collectedbythe target anode, and will pass. into the output circuit as current impulses which will tend to destroy the relationship between the selected electrons, and the illumination of that area of the cathode where they originated. Thus the output current would be adulterated by current impulses which serve no useful function, and only serve to distort and perhaps obliterate the reproduced image.

In order to prevent these troublesome secondary electrons from entering the picture current circuit. I provide a secondary collecting anode 88 adjacent the aperture i8 and in axial alignment therewith. This anode is supported by a lead 81 passing through a seal at one edge of the window 8 and a lead 88 is connected to the lead 81 and to the positive terminal of a battery 88. The negative terminal of this battery is connected to the positive terminal of the battery 82. Thus the collector anode is provided with a greater positive charge than the target anode or the sleeve anode l4.

As the relatively slow moving secondary electrons are easily attracted toward the electrode having the greatest positive charge, they will all tend to flow to the collector anode 88, thereby eliminating the possibility of their entering the output circuit through the target anode and adulterating the picture current. The collector anode will not substantialy affect or divert the primary electrons of the beam, as they traverse their path from the'cathode to the target anode, due to the restraining force of the focusing and deflecting fields and to their relatively high velocity.

Figures 2 and 3 are diagrammatic views showing modified arrangements of the tube electrodes and their circuit connections. In Figure 3 the collector anode 4| is shown as a ring located directly adjacent the aperture It. By placing the anode in this position it exerts a greater attracting force on the secondary emission from the target anode and prevents the passage of secondaries from the sleeve anode through the aperture. It will, however, offer no impediment to the passage of the high velocity electrons, for the reason that they are moving at such a speed that they pass through the collector anode before they can swerve and be intercepted thereby.

Figure 2 shows a modified circuit arrangement in which the target anode 28 is the most positively charged of the electrodes.

The secondary collecting anode, in this case, is disposed between the aperture and the target anode and is negatively charged with respect to these electrodes. As secondaries are liberated from the target anode, the negative charge on the secondary anode repels them, and drives them back toward the target anode. Similarly, any secondaries which tend to enter the aperture from the sleeve anode, are repelled by the negatively charged secondary anode, and return to the positively charged sleeve anode. As described above, the passage of the high velocity primary electrons to the target anode is not afiected in any way.

From the foregoing description of my invention it will be seen that I have provided efiective and simple means for controlling secondary emission in electron discharge devices by preventing the admittance thereof into circuits where it serves no useful function. While I have chosen to illustrate my invention as being incorporatedin an image dissector tube for television use, it is understood that it may be used in cathode ray tubes or other electron discharge devices within the scope of the claims.

I claim:

1. An image dissector tube having a planar photoelectric cathode adapted to initiate a discharge of cathode rays having an elemental cross sectional density corresponding to the intensity of light of elemental areas of an image projected thereon, means for causing portions of the discharge to follow mean rectilinear paths and means for causing other portions of the discharge to follow mean curvilinear paths, an anode for collecting successive portions of the discharge following said mean rectilinear paths and a second anode for collecting those portions of the dis-' charge following said mean curvilinear paths, and means disposed between the anodes for preventing the passage of secondary electronic emission from one of the anodes to the other thereof.

2. A cathode ray tube having a cathode adapted to discharge a beam of electrons, a compound anode adapted to receive separate portions of said beam comprising, a tube having an aperture therein, a conductor disposed within the tube, and a ring disposed between the tube and the conductor.

3. A cathode ray tube having means for emitting a beam of electrons and a plurality of means for collecting portions of said beam, and means interposed between said collecting means for repelling secondary electrons emitted by one of said collecting means back to said collecting means.

RUSSELL H. VARIAN. 

